Obama chides Putin over separatists, nuke treaty

US President Barack Obama called his Russian counterpart Vladimir Putin on Friday to express his "deep concerns about Russia's increased support for the separatists in Ukraine," the White House said.

Obama also repeated his concern about Moscow's alleged breach of the 1987 Intermediate-Range Nuclear Forces Treaty, under which the US and Russia agreed not to develop medium-range cruise missiles.

"The president reinforced his preference for a diplomatic solution to the crisis in Ukraine, and the two leaders agreed to keep open their channels of communication," a White House statement said.

Separately, the Kremlin said the two leaders had agreed that the current situation in Ukraine -- where pro-Russian separatists are battling government -- was not in either country's interest.

Putin also, according to the Kremlin, told Obama Western economic sanctions were "counterproductive, causing serious damage to bilateral cooperation and international stability overall."

Pentagon spokesman Rear Admiral John Kirby meanwhile said Russia was continuing to reinforce its military presence along the border with Ukraine.

"It continues to be north of 10,000, the numbers, but it fluctuates," Kirby told reporters, adding that troops were "close to the border, within 50 kilometers of the border -- closer than what we saw back in the spring."